In old times, incandescent light bulb is a major lighting equipment. But its efficiency is low. Lots of energy is wasted in the form of heat. So, the global trend of the lighting equipment is from incandescent bulb to CFL bulb, and then to LED bulb. The CFL bulb includes mercury which is toxic. The LED bulb is most energy efficient but there exists a LED flickering problem.
For incandescent bulbs and CFL bulbs, the light tube also has light flickering, but the problem for LED bulbs is more serious. If people are exposed to bulb which has invisible frequency (high frequency) flicker, it will trigger malaise, headaches and impaired visual performance. Also, if people are exposed to visible frequency (low frequency) flicker, it will trigger epileptic seizure, headaches and impaired visual performance.
Nowadays, a LED lamp is becoming a very common type of lighting equipment. If the user has a simple way to distinguish the extent of LED flickering, they can avoid using the lamp which is harmful to their health.
An actual lamp produces flickers which are rapid and repeated changes over time in the brightness of light, i.e. brightness waveform with a fixed frequency. For LED lamps, it is caused from the rapid and repeated change over time in current of its driver, which is an unavoidable phenomenon.